Cancel in-progress workflow runs for pull requests and dispatches (#16601)

This will reduce unnecessary resource consumption by cancelling running workflows when new commits are pushed to a PR or workflow dispatch. It does not cancel pushes to main or branches.

Closes #16601

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/16601 from sethvargo:sethvargo/actions-concurrency 82fd070dba
PiperOrigin-RevId: 629805311

.github/workflows/test_runner.yml

@@ -52,6 +52,10 @@ on:
 permissions:
   contents: read
 
+concurrency:
+  group: ${{ github.event_name }}-${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: ${{ contains(fromJSON('["pull_request", "pull_request_target", "workflow_dispatch"]'), github.event_name) }}
+
 jobs:
   check-tag:
     name: Check for Safety